Process of resolving dl-serine m-xylene-4-sulfonate

ABSTRACT

A supersaturated solution of DL-serine m-xylene-4-sulfonate is prepared. The solution is seeded with crystals of one of the optically active enantiomers of said DL-serine m-xylene-4sulfonate before and/or after the solution reaches the point of supersaturation. Crystallization is permitted to take place and the resultant crystals are recovered. The resulting enantiomer is useful as an intermediate in preparing optically active serine.

United States Patent [1 1 Chibata et al.

PROCESS OF RESOLVING DL-SERINE M-XYLENE-4-SULFONATE [75] Inventors:Ichiro Chibata, Suita-shi, Osaka-fu;

Shigeki Yamada, Toyonaka-shi, Osaka-fu; Masao Yamamoto, Kyoto-fu, all ofJapan [73] Assignee: Tanabe Seiyaku Co., Ltd., Osaka,

Japan [22] Filed: Oct. 2, 1969 [21] Appl. No.: 863,375

[30] Foreign Application Priority Data Oct. 4, 1968 Japan 43/72249 Oct.4, 1968 Japan 43/72250 [52] US. Cl. 260/501.12, 260/534 M [51] Int, Cl.C07c 143/28 [58] Field of Search 260/50l.12

[ 1 June 26, 1973 [56] References Cited UNITED STATES PATENTS 13,523,969 8/1970 Chibata et al. 260/501.l2

Primary Examiner-Leon Zitver Assistant Examiner-Michael W. GlynnAttorney-Harry C. Bierman, Jordan B. Bierman and Bierman and Bierman [57] ABSTRACT I 10 Claims, No Drawings PROCESS OF RESOLVING DL-SERINEM-XYLENE-4-SULFONATE This invention relates to the resolution ofDL-serine m-xylene-4-sulfonate by selective crystallization, andprovides a novel process for the preparation of optically active serine.

Naturally occuring serine is in the optically active L- form. It is animportant component of animal diets, wh-

ereas the enantiomorphic D-serine has no known nutritional value.However, D-serine is very useful as an intermediate for the synthesis ofthe antibiotic D- cycloserine.

Synthetic serine is optically inactive and consists of equal parts ofthe two enantiomorphic isomers. Thus the commercial value of syntheticserine could be doubled if the isomeric mixture were resolved into D-and L-serine.

The most commonly used method of resolving DI..- serine comprisestreating an acyl-DL-serine with an optically active resolving agent suchas brucine or quinine, fractionally crystallizing the resulting mixtureor diastereoisomers and hydrolyzing the product. Alternatively, theN-acyl-DL-serine can be asymmetrically hydrolyzed by the action ofacylase. But these methods have been found to be disadvantageous as theyrequire the use of expensive alkaloids or the preparation of the enzyme.

A racemic modification of an organic compound can generally be resolvedby selective crystallization into each of its optically activeenantiomers if said racemic modification exists in the form of a racemicmixture. However, it is impossible to predict whether or not a givenracemic modification has such beneficial properties and whether or notresolution of a given racemic modification is possible. Only rarecompounds satisfy those requirements. Therefore, racemic modificationmust be further studied experimentally to determine whether selectivecrystallization can be accomplished. Although the method of selectivecrystallization is useful for the commercial production of opticallyactive amino acid, DL-serine itself can be resolved by such a method.

It has now been found that, when DL-serine is converted into DL-serinem-xylene-4-sulfonate (which is hereinafter designated as DL-serinemxylenesulfonate), the said salt has many beneficial properties whichmake it suitable for selectively crystallizing it into each of itsoptically active enantiomers. Namely, the racemic modification of serinemxylenesulfonate is more soluble than both of its enantiomers; asaturated solution of the racemic modification will not dissolve anymore of the individual enantiometer; the supersaturated solution of anenantiomer is stable even after the selective crystallization of theother optically active enantiomer; prompt crystallization of each of theenantiomers is afforded; and an optically active enantiomer of serinem-xylenesulfonate can be selectively crystallized out of asupersaturated solution of the racemic modification or from asupersaturated solution containing the racemic modification and one ofthe enantiomers.

Anobject of this invention is, therefore, to provide an economical andcommercially useful process for the resolution of DL-serinem-xylenesulfonate into each of its enantiomers. The process of thepresent invention is free of the disadvantages of the abovementionedknown methods. Further, according to the process of the presentinvention, the yield of a desired optically active serinem-xylenesulfonate may be remarkably increased due to the suitablesolubility of said salt in water. Another object of this invention is toprovide a novel method for the industrial production of optically activeserine prepared from optically active serine mxylenesulfonate. Otherobjects of this invention will be apparent from the followingdescriptions and claims.

The process, according to the present invention, comprises producing asupersaturated solution of DL- serine m-xylenesulfonate in a solvent,seeding or dissolving one of the optically active enantiomers thereofinto the solution thus making it predominant over the other enantiomerin the solution; allowing the predomi nant enantiomer to crystallizeout; and then recovering it from the solution.

In one embodiment of the invention, a small amount of crystals of one ofthe enantiomers is added to the supersaturated solution as a seed andthe mixture is stirred to cause selective crystallization of theenantiomer which is the same as that which was seeded. Alternatively, asmall amount of one of the enantiomers is dissolved in a hot solution ofthe racemic modification in order to make the said enantiomer dominantover the other in the solution. The solution is then cooled wherebyspontaneous crystallization of the enantiomer which is the same as thatwhich was added takes place. It is also possible to combine theseprocedures. Namely, a partial amount of the crystals of one of theenantiomers is dissolved in the solution of the racemic modification andthe remaining part is used to seed the supersaturated solution in whichone of the enantiomers is dominant over the other. In this case, theseeding amount can be minimized. The supersaturated solution may beprepared from a solution of serine mxylenesulfonate in a suitablesolvent using conventional procedures as for example refrigeration,concentration, addition of appropriate solvents or a combination ofthese operations. For the preparation of the supersaturated solutionthereof, however, it is most convenient to cool a hot solution saturatedwith serine mxylenesulfonate, as the solubility thereof increases withan increase in the temperature.

The seed-crystals to be used should have a high optical purity. However,the equivalent mixture, of D- and L-enantiomers need not always be usedas the starting material for the resolution. The non-equivalent mixturethereof can also be used for this purpose. lt is rather convenient touse the non-equivalent mixture thereof as the starting material of thepresent invention, because the predominant enantiomer in the mixture maybe spontaneously crystallized out from the supersaturated solution ofsaid material. The preferred amount of the seed to be added is about 0.1percent by weigh based on the weight of the solution. It should be notedthat the greater the amount of the seed, the better the resultantresolution. If the solution already contains an optically activeenantiomer dominant over its antipode because of the natural occurenceof the seed crystals, the need for seeding with seed crystals of theoptical enantiomer which is dominant over the other is obviated.Nevertheless, for smooth resolution seeding is preferred. Although thetemperature at which the crystallization 'is carried out is not criticalfor the method of the invention, a temperature of about room temperatureis preferred. Any inert solvent which can dissolve DL-serinem-xylenesulfonate and which can crystallize out the compound as aracemic mixture is suitable in the process of selective crystallization.Water, an aqueous solvent, for example, a solution containing alkanolshaving up to six carbon atoms or an alkanone having up to six carbonatoms are suitable for this purpose. From an industrial standpoint wateris the most suitable solvent. After one of the optically activeenantiomers has been crystallized out and separated from the motherliquor, the other enantiomer remaining in said mother liquor becomesdominant over the enantiomer which was crystallized out. The motherliquor may again be employed for the optical resolution of the otherenantiomer. For this purpose in order to produce the supersaturatedsolution of the enantiomorphic mixture, the mother liquor isconcentrated. Alternatively, a quantity of the racemic modification,which quantity is preferably equal to the amount of the enantiomerpreviously separated, may be dissolved in the mother liquor. The

procedure which was carried out in the previous operation is thenrepeated to separate out the other enantiomer. In this case, if theamount of the racemic modification to bev added is adjusted equal to theamount of the enantiomer previously separated, the same condition withthe previous operation can be afforded except that the predominantenantiomer in the solution is the antipode of the enantiomer previouslyseparated. Thus the cycle of the operation may be preferably repeated,whereby the racemic modification supplied may be successively andentirely resolved into each of the D- and L-enantiomers.

The process of the present invention can be carried out not only by thebatch system as mentioned above, but also by the continuous system whichcomprises, for example, the'steps of passing the supersaturated solutionthrough'a column containing the seed crystals, and allowing an opticallyactive serine m-xylenesulfonate to crystallize out selectively in thesaid column. Alternatively, the process of the present invention can-becarried out'by immersing the seeding plates of opticallyactive-'enantiorners in the supersaturated solution and allowing theoptical enantiomers to crystallize out on the seeding plates.

The resultant crystals thus obtained may sometimes be optically impuredue to the degree of supersaturation and the amount of crystalsseparated. The crude crystals, however, may be easily purified, becausethe solubility of the racemic modification is sufficiently higher thanthat of each enantiomer and the said optically active enantiomer will-bedissolved in the saturated solution of the racemic modification. Namely,the optically pure crystals of serine m-xylenesulfonate can be obtainedby adding the crude crystals into a minimal amount of solvent willdissolve the racemic modification in the said crude crystals, stirringthe solution and then recovering the resultant crystals from thesolution.

' Alternatively, the optically pure crystals of serine mxylenesulfonatecan be obtained by dissolving the crude crystals, for example, at anelevated temperature in a small amount of solvent which dissolves theracemicmodification in the crude crystals, allowing the said enantiomerto crystallize out and recovering it from the solution.- Such operationsas refrigeration, concentration, the addition of a solvent orcombinations thereof may be used for crystallization of the opticallyactive enantiomer from the solution. Any inert solvents which aredescribed above may also be used for this purpose.

When only a small amount of solvent is needed due to the low content ofthe racemic modification in said crude crystals or the high solubilityof the racemic modification, it is convenient to carry out the aboveoperation by adding-a suitable amount of the solution saturated withDL-serine m-xylenesulfonate.

Optically active serine can be obtained from optically active serinem-xylenesulfonate without racemization by treating with ion-exchangeresin and liberating the resultant optically active serine.

DL-serine m-xylenesulfonate, a starting compound of the presentinvention can be prepared by neutralizing DL-serine withm-xylene-sulfonic acid in a suitable solvent.

Serine m-aminoxylenesulfonate is a novel compound in both the racemicmodification form and the optically active form. The physical propertiesof serine m-aminoxylenesulfonate are shown in the following Tables.

TABLE I Serine m-xylene- M.p. Specific rotation sulfonate (*C.) C= 4,water) DL-fonn l57-l58 0 D-form l72l73 4.05 L-form l72 l73 +4.05

TABLE II Temperature Solubility g./ 100g. of water) (C. L-form DL-form15 23 .5 45 l 25 39.5 80.9 40 86.8 i 75 .4

As will be apparent from the data represented by Tables l II, serinem-xylenesulfonate has beneficial properties suitable for the selectivecrystallization of the racemic modification into each of its opticallyactive enantiomers.

EXAMPLE 1 1 14.0 g. of DL-serine m-xylenesulfonate are dissolved in 100ml. of water under heating. The solution is then cooled to 30C. 4.0 g.of L-serine mxylenesulfon'ate 2 hydrate is seeded into the solution. Themixture is stirred for 65 minutes at the same temperature. Then theresultant crystals are collected by filtration. The crystals thusobtained are washed with a small amount of cool water, and acetone anddried at 50C. under ventilation whereby 9.5 g. of L-serinemxylenesulfonate are obtained. [04,, 4.05 C 4, H O Optical purity: 100percent Nitrogen analysis calculated: 4.81 Found: 4.76

6.0 g. of the crystals are dissolved in 60 ml. of water and the solutionis passed through a column of 20 ml. of ion-exchange resin (AmberlitelR-l20, l-l-form.). The column is washed with water and eluted with lN-aqueous ammonia. The eluate is concentrated and methanol is added to theresidue to yield 2.1 g. of L- serine. [041 14.6 C 2, N-HCl Nitrogenanalysis: Calculated: 13.33 Found: 13.37

EXAMPLE 2 58.2 g. of DL-serine m-xylenesulfonate and 1.8 g. of D-serinem-xylenesulfonate are dissolved in 50 ml. of water under heating andthen cooled to 30C. 0.1 g. of D-serine m-xylenesulfonate is seeded intothe solution. The mixture is stirred for minutes at the sametemperature. The resulting crystals are collected by filtration. Thecrystals thus obtained are treated in the same [od l4.4 C 2, lN-HCI)EXAMPLE 3 47.0 g. of DL-serine m-xylenesulfonate are dissolved in 50 ml.of water under heating and then cooled to 25C. 50 mg. of D-serinem-xylene-sulfonate is seeded into the solution. The mixture is stirredfor 50 minutes at the same temperature. The resultant crystals arecollected by filtration whereby 6.3 g. of D-serine mxylenesulfonate areobtained.

percent 6.9 g. of DL-serine m-xylenesulfonate are dissolved underheating in the mother liquor obtained in the above operation. Thesolution is cooled to 25C. and 50 mg. of L-serine m-xylenesulfonate isseeded into the solution. The mixture is-stirred for 50 minutes at thesame temperature. The resultant crystals are treated in the same manneras described in Example 1 to yield 6.1 g. of L-serine m-xylenesulfonate.

[011 3.95( C 4, H O Optical purity: 97.5

percent Optical purity: 100

EXAMPLE 4 35.0 g. of DL-s'erine m-xylenesulfonate and 1.5 g. of D-serinem-xylenesulfonate are dissolved in 25 ml. of

aqueous ethanol (20 percent v/v) under heating. Thesolution is thencooled to30C. 50 mg. of D-serine mxylenesulfonate is seeded into thesolution. The mixture is stirred for 50 minutes at the same temperature.The resultant crystals are collected by filtration whereby 4.1 g ofD-serine m-xylene-sulfonate are obtained. [011 3.93 C 4, H O Opticalpurity: 97.0 percent EXAMPLE 5 23.5 g. of DL-serine m-xylenesulfonateand 1.5 g. of D-serine m-xylenesulfonate are dissolved in ml. of aqueousacetone (50 percent v/v) under heating. The solution is then cooled to C50 mg. of DL-serine mxylenesulfonate is seeded into the solution. Themixture is stirred for 40 minutes 'at the same temperature.

The resultant crystals are collected by filtration whereby 3.3 g. ofD-sei'ine m-xylenesulfonate are obtained.

[01],, 3.95 C 4, H 0 Optical purity: 97.5

percent EXAMPLE 6 EXAMPLE 7 50 g. of L-serine m-xylenesulfonate (opticalpurity: 92.6 percent) are added to a mixture of 3.6 ml. of water and ml.of a solution saturated with DL-serine mxylenesulfonate at 30C. Themixture is then heated until solution is complete. After cooling to30C., the solution is stirred for 1.5 hours. The resultant crystals arecollected by filtration, washed with a small amount of water, acetoneand dried whereby 45.6 g. of L-serine m-xylenesulfonate are obtained.

[01],, +4.05 C 4, H O Optical purity: 100

percent EXAMPLE 8 EXAMPLE 9 13.5 g. of L-serine rn-xylenesulfonate(optical purity: 61.7 percent) are added to a mixture of 4.4 ml. of 20percent (v/v) aqueous ethanol and 15 ml. of asolution saturated withDL-serine m-xylenesulfonate in 20 percent (v/v) aqueous ethanol at 30C.The mixture is then heated until solution is complete. The solution isstirred 'for two hours at 30C. The resultant crystals are collected byfiltration, washed with a small amount of 20 percent (.v/v) aqueousethanol, acetone and dried whereby 8.1 g. of L-serine m-xylenesulfonateare obtained.

[011 +4.05 C 4, H O Optical purity: 100

percent EXAMPLE l0 10 g. of D-serine m-xylenesulfonate (optical purity:72.8 percent) are added to a mixture of 2.6 ml. of 50 percent (v/v)aqueous acetone and 20 ml. of a solution saturated with DL-serinem-xylenesulfonate in 50 percent (v/v) aqueous acetone at 30C. Themixture is then heated until solution is complete. The solution isstirred for 2.5 hours at 30C. The resultant crystals are collected byfiltration, washed with a small amount of acetone and dried whereby 7.1g. of D-serine mxylenesulfonate are obtained. Optical purity: 100percent i What is claimed is:

l. A process for resolving DL-serine m-xylene-4- sulfonate into itsoptically active enantiomers which comprises the steps of addingcrystals of one of said enantiomers to a solution of DL-serinem-xylene-4- sulfonate in water, a mixture of water and an alkanol havingup to six carbon atoms, or a mixture of water and an alkanone having upto six carbon atoms and supersaturating said solution wherebycrystallization of said one of said enantiomers from the solution isinitiated then recovering the crystallized one of said enantiomers.

2. A process as claimed in claim 1 wherein said one of said enantiomersis added as seed crystals to the supersaturated solution of DL-serinem-xylene-4- sulfonate.

3. A process as claimed in claim 2 in which the amount of the seedcrystals added is about 0.1 weight percent based onthe weight of thesolution.

4. A process as claimed in claim 1 wherein said one of said enantiomersis added to the solution of DL- serine m-xylene-4-sulfonate at anelevated temperature then the DL-serine m-xylene-4-sulfonate solution istals of said one of said enantiomers.

6. A process as claimed in claim 1 further including the steps ofdissolving, at elevated temperature, additional DL-serinem-xylene-4-sulfonate in mother liquor obtained after the recovery ofsaid crystallized one of said enantiomers thereby producing anothersupersaturated solution, allowing crystallization to take place andrecovering the crystallized other one of said enantiomers.

7. A process as claimed in claim 6, wherein said process is repeated aplurality of times whereby said optically active enantiomers aresuccessively and alternatively separated as crystals from the solutionof DL- serine m-xylene-4-sulfonate.

8. DL-serine m-xylene-4-sulfonate.

9. D-serine m-xylene-4-sulfonate.

10. L-serine m-xylene-4-sulfonate.

2. A process as claimed in claim 1 wherein said one of said enantiomersis added as seed crystals to the supersaturated solution of DL-serinem-xylene-4-sulfonate.
 3. A process as claimed in claim 2 in which theamount of the seed crystals added is about 0.1 weight percent based onthe weight of the solution.
 4. A process as claimed in claim 1 whereinsaid one of said enantiomers is added to the solution of DL-serinem-xylene-4-sulfonate at an elevated temperature then the DL-serinem-xylene-4-sulfonate solution is cooled to produce said supersaturatedsolution.
 5. A process as claimed in claim 1 wherein said one of saidenantiomers is added to the solution of DL-serine m-xylene-4-sulfonateat an elevated temperature then the DL-serine m-xylene-4sulfonatesolution is cooled to produce said supersaturated solution and thesupersaturated solution is innoculated with seed crystals of said one ofsaid enantiomers.
 6. A process as claimed in claim 1 further includingthe steps of dissolving, at elevated temperature, additional DL-serinem-xylene-4-sulfonate in mother liquor obtained after the recovery ofsaid crystallized one of said enantiomers thereby producing anothersupersaturated solution, allowing crystallization to take place andrecovering the crystallized other one of said enantiomers.
 7. A processas claimed in claim 6, wherein said process is repeated a plurality oftimes whereby said optically active enantiomers are successively andalternatively separated as crystals from the solution of DL-serinem-xylene-4-sulfonate.
 8. DL-serine m-xylene-4-sulfonate.
 9. D-serinem-xylene-4-sulfonate.
 10. L-serine m-xylene-4-sulfonate.